[Elecraft] Where to put the wattmeter

[Dave New, N8SBE]

Bob,

I might agree with you, EXCEPT I was using the setup with the P3 2KW 
sensor between the KAT500 and the antenna for several months or more, 
before the P3 crapped out on me.  And during that time, I didn't notice 
any unusual power readings.  This may be because the sensor outputs 4 
separate signals, VFWD, VREV, VSCL, and VREF.

So far, I've not been able to get anyone to explain to me the use of the 
VSCL and VREF signals, but if (e.g.) the VREF signal may serve the same 
purpose as I used to use a full-scale 'set' point on an SWR meter, which 
would take care of any forward power differences.

So, who designed the original W2 wattmeter, where these sensors were 
first used, and can they explain the usage/formulas use with those four 
signals?  There is no theory of operation in the downloadable W2 manual.

73,
-- Dave, N8SBE

On 2023-04-13 12:20, Bob McGraw wrote:
> While I totally agree with Alan, N1AL, I fear that one who is reading
> an external power meter located in the feedline after the tuner will
> be mislead by the power indications.  At the same time, SWR
> indications are typically valid.
> 
> Case and point.   On one of my antennas, on a frequency where the SWR
> is high, the FWD power indicated on the external meter is 200 watts. 
> No way the radio is producing 200 watts.  At the antenna matched
> frequency the external meter indicates 100 watts.
> 
> The original post indicated with the OPs K3S the P3 Sensor located
> after the KAT500 antenna tuner was showing a power value of some 300
> watts or so.  Even more non believable values with his KPA500.  Thus
> my reason for questioning the position of the measuring sensor in the
> feed line.   Most power measurement devices are calibrated for a known
> impedance, normally 50 ohms. Therefore placing them in the feedline
> where an unknown impedance exist, one will also have a false power
> indication.
> 
> Exceptions are a calculating power meter that measures both voltage
> and current to determine power.  Most power meters I find in ham usage
> are only voltage sensing devices.
> 
> That's all.
> 
> 73
> 
> Bob, K4TAX
> 
> 
>> Date: Mon, 10 Apr 2023 10:03:10 -0600
>> From: Alan Bloom<n1al at sonic.net>
>> To:elecraft at mailman.qth.net
>> Subject: Re: [Elecraft] Where to put the wattmeter
>> Message-ID:<f792d09d-367a-7ff3-eec0-10d84f20bad2 at sonic.net>
>> Content-Type: text/plain; charset=UTF-8; format=flowed
>> 
>> I changed the Subject line since this is a new topic.
>> 
>> If the wattmeter (in this case the P3 sensor) is placed after the 
>> tuner
>> then it will read the SWR of the antenna.? To determine the net 
>> forward
>> power, just subtract the reflected power from the forward power.
>> 
>> To calculate reflected power, use the equation:
>> 
>> P(ref) = P(fwd) * [(SWR-1)/(SWR+1)]^2
>> 
>> Since the K3 and K4 already have a built-in wattmeter located before 
>> the
>> tuner, it makes sense to me to put the P3 sensor (or external 
>> wattmeter)
>> after the tuner so you can see what is going on with the antenna.
>> 
>> Alan N1AL
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to n8sbe at densbe.com